The purpose of the research proposed here is to discover the determinants of the seasonal fluctuations in births in the United States and other countries. Although seasonal fluctuations account for approximately half of the non-trend variation in births, there are not generally accepted explanations for this seasonality. We believe determining the key causes of the seasonal in births is crucial both because seasonality constitutes a major source of birth fluctuations and because isolating the determinants of seasonality in births will increase our understanding of the determinants of births in general. The first step in our research will be to document the seasonality of births across countries and time using a consistent and well specified empirical framework. The purpose of this step in the research is to establish clearly what the facts are about seasonality in births. This provides a logical starting point for formulating hypotheses to explain these facts, and it also serves a useful purpose in its own right by unifying the existing literature on seasonality. The next step in the research will be to provide a theoretical foundation for evaluating empirically alternative explanations of seasonality in births. The proposal provides an example of such a model and shows that even with a simple model one can derive important testable implications for the data. The remaining part of our research will test the implications of various explanations of the seasonal in births, using our theoretical model as a framework for interpreting the results of the tests. Using an econometric approach specified in the proposal, our research will identify the direct contribution of weather in explaining monthly births. Our research will also test hypotheses about the relationship between contraception, abortion and seasonality. The theoretical model presented below shows that the introduction of a highly effective contraceptive, such as the pill, will have different effects on the seasonality of births depending on the source of the seasonality. Examining seasonality across 'contraceptive regimes' is therefore a way to test for the importance of different explanations of the seasonality.